A common industrial process for producing an amide compound comprises producing an oxime compound from a corresponding ketone and hydroxylamine followed by Beckmann rearrangement of the oxime compound. For example, ε-caprolactam, which is industrially useful, is produced by Beckmann rearrangement of cyclohexanone oxime. The rearrangement is generally conducted using concentrated sulfuric acid and oleum, and since these strong acids must be used in the stoichiometric amounts or more, salts such as ammonium sulfate are formed as byproducts during neutralization in a far larger amount than that of the product ε-caprolactam. In other words, the process requires many facilities and large amounts of starting materials and energy for producing a large amount of auxiliary materials such as sulfuric acid and treating byproducts such as sodium sulfate.
In contrast, a liquid-phase Beckmann rearrangement reaction using a catalyst requires no auxiliary materials and produces less byproducts, and is, therefore, expected to be put to practical use. A Beckmann rearrangement catalyst in a liquid phase has been extensively studied. For example, an aromatic-ring-containing compound disclosed in Patent Reference No. 1 can be used, which (1) contains, as an aromatic-ring member, at least one carbon atom having a leaving group and (ii) contains at least three aromatic-ring members which are either or both of heteroatoms or/and carbon atoms having an electron-withdrawing group, and (iii) wherein, two of the heteroatoms and/or carbon atoms having an electron-withdrawing group are at the ortho- or para-position to the carbon atom having a leaving group. Examples of a practically used Beckmann rearrangement catalyst include trichlorotriazine (also known as cyanuric chloride, 2,4,6-trichloro-1,3,5-triazine, abbreviated as “TCT”) and hexachlorophosphazene (Patent Reference No. 13).
Patent Reference No. 2 has described that a Beckmann rearrangement reaction is conducted in a nonpolar solvent using a catalyst disclosed in Patent Reference No. 1. Patent Reference Nos. 3 and 4 have described a process of Beckmann rearrangement of an oxime compound using an analogous compound to a catalyst disclosed in Patent Reference No. 1. Patent Reference Nos. 5 and 6 have disclosed Beckmann rearrangement of an oxime compound using an acid chloride such as thionyl chloride as a catalyst.
Patent Reference Nos. 7 and 8 have disclosed a specific process for producing an amide compound by Beckmann rearrangement using a catalyst disclosed in Patent Reference No. 1, but have not specifically described recycling of a solvent and the like.
Lactams are mainly used for polymers or copolymers for yarns, fibers, films and the like, and in some cases must have purity satisfying strict standard values.
Examples of typical standard values include those based on an absorbance such as a light transmittance difference (LT. diff, detailed later), a UV value (a UV absorbance (at a wavelength 290 nm) of a 50% by weight aqueous solution of a lactam is measured within a cell having a width of 1 cm), and a PAN value (ISO standard 8660).
As a method for improving a UV value and a PAN value among these standard values, many techniques have been disclosed, including distillation, solvent washing, crystallization/recrystallization, acid treatment, alkali treatment, oxidation, hydrogenation and the like of a lactam product. For example, Patent Reference No. 14 has described that a PAN value can be improved by preventing a nickel catalyst from entering a reboiler in hydrogenating a caprolactam over a nickel catalyst followed by purification by distillation.
Patent Reference No. 15 has disclosed influence of impurities in cyclododecanone as a starting material on an LT. diff of laurolactam that is a desired compound.
A known method for achieving a low UV value is hydrogenation of a lactam in the presence of a catalyst. For example, Patent Reference No. 9 has disclosed a method for hydrogenating a caprolactam prepared by a Beckmann rearrangement reaction, in the presence of a suspended hydrogenation catalyst. Furthermore, Patent Reference Nos. 10 and 11 have disclosed a method for hydrogenating a caprolactam after treatment with activated charcoal and an ion-exchange resin. Patent Reference No. 12 has disclosed a method for hydrogenating a lactam prepared by cyclization hydrolysis of an aminonitrile, in the presence of a hydrogenation catalyst.
The above techniques relate to improvement in the standard values of a lactam prepared by Beckmann rearrangement of an oxime using a strong acid such as sulfuric acid. There have disclosed no purification methods for improving the above standard values for a lactam prepared by a production process without a large amount of ammonium sulfate as a byproduct of a Beckmann rearrangement reaction being produced. Furthermore, the above disclosed techniques show relationship between a treatment method and UV value, PAN value or the like, but do not identify substances causing deterioration in standard values or not show relationship between the concentrations of the causative substances and the standard values.
On the other hand, while the above standard values are effective only for the case where an impurity in an amide compound shows a given UV absorption or reacts with potassium permanganate, it cannot contribute to detection of other impurities.
The presence of a lactam other than a desired lactam and an amide compound is undesirable because they may inhibit polymerization and deteriorate physical properties of a polymer. However, when there are, as impurities, a lactam and/or an amide compound having a structure different from that of a product lactam, the above standard values are not always changed, so that it is necessary to establish a method for detecting and assaying the impurities and measures for reducing them.